1. Field of the Invention
The present invention relates to piezoelectric transformers which use piezoelectric members.
2. Description of the Related Art
A piezoelectric transformer includes an elongated plate-type piezoelectric member including first and second electrodes disposed opposite to each other in a thickness direction of the piezoelectric member. The first and second electrodes are disposed on a first portion of the piezoelectric member at a first end in a longitudinal direction of the piezoelectric member. The piezoelectric member is polarized in a thickness direction at the first portion of the piezoelectric member at which the first and second electrodes are disposed. A third electrode is located on the end surface of a second portion of the piezoelectric member including a second end disposed opposite to the first end in the longitudinal direction, and the piezoelectric member is polarized in the longitudinal direction at the second portion at which the third electrode is located. The first portion having the first and second electrodes disposed opposite to each other in the thickness direction functions as a driving section or an input section. The second portion functions as a power generating section or an output section.
When an input voltage having a natural frequency determined by the dimension of the piezoelectric member in the longitudinal direction is applied between the electrodes of the driving section, which are disposed at the first portion of the piezoelectric member, the piezoelectric member generates strong mechanical vibration in the longitudinal direction due to an electrical deformation effect. Then, the piezoelectric member generates charges at the second portion functioning as the power generating section due to the piezoelectric effect. These charges flow as an alternating high voltage between one of the first and second electrodes in the driving section and the third electrode of the power generating section. In this way, the piezoelectric transformer causes a voltage rise through a conversion from electrical energy to mechanical energy and then a conversion from the mechanical energy to electrical energy.
In such a piezoelectric transformer, a piezoelectric ceramic material such as lead zirconate titanate is usually used to form the piezoelectric member.
As a piezoelectric transformer having a relatively large voltage-rise ratio, a laminated piezoelectric transformer has been proposed. The laminated piezoelectric transformer has a piezoelectric member made up of a laminated-layer member including a plurality of piezoelectric ceramic layers. A plurality of driving electrodes serving as input electrodes are formed such that an electrode opposes an adjacent electrode or adjacent electrodes through the piezoelectric ceramic. On external surfaces of the laminated-layer member, driving terminal electrodes and a power-generating terminal electrode are located. A plurality of driving electrodes are alternately and electrically connected to the opposite terminal electrodes. The power-generating terminal electrode serves as an output electrode.
Since the driving section and the power-generating section are polarized by using electrodes extending in the thickness and longitudinal directions for input and output in the above-described piezoelectric transformer, the degree of polarization is uniform both in the driving section and the power-generating section.
As a result of this arrangement, internal stress is generated at the boundary between the driving section and the power-generating section, in which the directions of polarization differ. The internal stress may cause the piezoelectric member to be destroyed when polarized or one or more small cracks in the piezoelectric member may occur as a result of polarization thereby reducing the mechanical strength of the piezoelectric member.